I was reading about telephone lightning arrestors in my ARRL RFI Book (first edition, copyright 1998), in chapter 9 "Telephone RFI" (by Pete Krieger, previously WA8KZH, now K8COM) and on page 9.6, the following is stated (emphasis mine):

The Lightning Arrestor

Telephone service enters a house at a grounded, fused lightning arrestor located outside at the house end of the telephone company drop. Years of exposure to weather or moisture can cause corrosion or discoloration (onset of corrosion) of wires, junction boxes or components inside the lightning protector housing. If the arrestor is accessible, a good visual inspection may reveal potential problems. Lightning arrestors, especially those that have done their job a few times, can become nonlinear, acting like a diode, rectifying any RF energy present on the phone lines (just like the crystal radios many hams built in their early years). If you discover that a lightning arrestor is creating RFI, the fact that it's rectifying RF is one indication that it needed to be replaced anyway! Modern arrestors are less prone to RFI problems than older ones.

Just a few notes about this quote... This is the entire (small) section, so this is likely all the information that I have behind my question. Also, the context does indicate that the lightning arrestor is standard telephone service equipment, and therefore part of a standard land-line telephone installation; therefore, this question may be limited to only standard telephone company equipment. Quite possibly, it may be a broader generalization -- I just don't know.

So, it's that last sentence that piqued my interest. Why, exactly, are modern lightning arrestors less prone to RFI problems? Is it more even "wearing" so it doesn't become a diode? Is it a better design, so it lasts longer and fails more gracefully?


I emphasized the "diode" aspects of the quote from the book a little more in preparation for making the following request:

I would like to argue that for me to accept an answer, your answer has to show either how semiconducting happened more frequently with the old style, or alternately show how semiconducting happens less frequently with the new style. This is because I can't really decide between the two really great answers already given. How did a lightning hit (or moisture, or time) cause carbon blocks (the old style) to semiconduct?

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    \$\begingroup\$ Welcome to ham.stackexchange.com! \$\endgroup\$
    – rclocher3
    Mar 25, 2020 at 13:46
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    \$\begingroup\$ I don't have that book, but "rectifying any RF energy present on the phone lines" sounds to me that it is talking about an arrestor installed by the telephone company on the phone lines. Does the context indicate that? Those older phone line arrestors used a stack of carbon blocks. \$\endgroup\$ Mar 25, 2020 at 16:00
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    \$\begingroup\$ @MikeWaters -- Yes, the context does indicate that the arrestor is installed by the telephone company, but carbon blocks are not mentioned. The Header is "The Lightning Arrestor", and the section begins, "Telephone service enters a house at a grounded, fused lightning arrestor located outside at the house end of the telephone company drop. Years of exposure to weather or moisture can cause corrosion or discoloration (onset of corrosion) of wires, junction boxes or components inside the lightning protector housing." Then, Pete Krieger, WA8KZH, indicates a visual inspection would be helpful. \$\endgroup\$ Mar 27, 2020 at 1:48
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    \$\begingroup\$ That sounds like an older book. In any case, you should add these details to your question, as comments are not searchable. \$\endgroup\$ Mar 27, 2020 at 2:09
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    \$\begingroup\$ @MikeWaters -- Done. \$\endgroup\$ Mar 27, 2020 at 2:52

4 Answers 4


The carbon block becomes exactly like a high power TRIAC (with a trigger voltage (<250V), when sufficient voltage and time duration for ionization exists.

There is an incremental negative resistance when triggered (avalanche effect) and is inversely proportional to the current.

Line inductance limits the Re-radiated EMI as it spews an impulse of low current yet widespread on the Telco line. The Broadband EMI is approx. L/-ESR = omega -3dB in a radial direction along to the CO. (Central office)

The rise time dI/dt=V/L is approx 0.35/f bandwidth using 80% of the risetime.(10~90%)

This parallel gap is closer to 3K/mm vs. a sharp edge or point =< 1kV/min dry air. This BVD threshold also degrades with air quality and higher humidity and surface contamination from sputtered carbon.

  • the above contributes to a much lower BDV threshold.
  • When contaminants are involved , there is always a canary in the mineshaft effect called partial discharge which sounds like static lightning when there is none. This also has a Relaxation Oscillator effect with random intervals due to ionization and ozone effects. So it might sound like a Geiger counter with self discharges rather than a linear effect of rectifying AM RF broadcasts, but this diode rectifying effect is also possible.
  • thus large epoxy coated metal oxide varistors are better, but also have a limited lifespan of conductions, so Floridians may need to have theirs replaced on a routine basis. This is based on past experience reading specs not handy at this time.

Using @Mike’s data 0.076 mm = 76V for 1kV/mm and 228 V @ 3kV/mm when new for the -48 to -54V telco DC voltage. thus to me is a good design value for an arrestor and also for a TVS but for an exposed carbon gap not so reliable long term. The relaxation pulses might not occur when the phone is off-hook low impedance but only when high impedance so the capacitance charge can build up from ingress of lightning, then partial discharge egress on the phone lines picked up by AM and HAM radios in addition to real lightning EMI hundreds of miles away which would be much stronger when close and weak far away. (Friis Loss)

If corona discharge on wet powerline bushing is say 100kV triggers the creapage moisture and dust at lower levels, so that when you walk near HV power lines and transformers, you may hear these in the morning with dew, like snap crackle pop which are harmless surface Partial Discharges (PD) But when sufficient dirt accumulates, it can lead to a failure just like carbon gap arrestors if not self cleaning from rain. I have heard HVAC lines crackle overhead in golf courses and back lanes of apartments. when they monitor this with circuits and determine a HVDC or HVAC line is at risk, they then fly helicopters and techs in faraday cage suits to spray clean the insulator bushings.

  • \$\begingroup\$ Couldn't they make the carbon rods a little longer, cap them with steel ends, and surround them with a glass envelope, and pot them in? To reduce their moisture-exposure and make them last longer. (Oops, probably make them explode, though... Hmmm...) \$\endgroup\$ Mar 30, 2020 at 15:42
  • \$\begingroup\$ And does the lightning going through the carbon rods make an intensely bright light, the same as in a search light? \$\endgroup\$ Mar 30, 2020 at 15:47
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    \$\begingroup\$ @MicroservicesOnDDD The negative resistance of carbon under high current could produce light easily with very low -ESR, but is normally limited by distance and line impedance which affects noise spectrum dI/dt=V/L andthe voltage if it was 2kV would also collapse to near 0 but introducing dV/dt=I/C with line capacitance. \$\endgroup\$ Mar 30, 2020 at 16:46

There are many different kinds of lighting arrestors -- and all of them work differently, age differently, and fail differently.

A lighting arrestor diverts the high voltage lighting strike to ground instead of allowing the voltage to pass through to your radio equipment.

Some are simply devices that are basically spark plugs. High voltage jumps across the electrodes and goes to ground while low voltage does not. Others consist of gas-filled tubes that ionize at high voltage and conduct while remaining inert and non conductive at lower voltages. Many today are solid state devices such as a series of MOVs.

After taking a lighting hit, the electrodes can be damaged or melted. The gas may heat up, expand, and create a leak due to the pressure. The junctions in the solid state devices may be damaged.

You can argue that the most dangerous failure is that they no longer send high voltage to ground, but there certainly are failure modes that can cause them to have high impedance, blocking RF, and in some cases rectifying it like a diode as they conduct part of the waveform to ground.

Many advise replacing the lighting arrestor after each lightning hit. Some also have an estimated life after which they should be replaced.

I don't know if I agree with a blanket statement that new lighting arrestor designs are better than older designs, but I would agree that a new manufactured arrestor is going to be better than one that has been in use for many years.


This is what an older telephone line lightning arrestor looks like.
Modern sealed metal-oxide varistors (MOVs) have replaced these, as they are a superior and more reliable design.

The protective cover at the top has been removed, exposing the two carbon blocks separated by a brass strip (which I believe to be either a varistor or a carbon block spark gap). Probably, the brass strip is connected to a ground rod, and the outer contacts on each block are respectively connected to each wire.

Those blocks may not be pure carbon, and I don't know what the two red cylinders are.

The one in my parents' former home had a porcelain base instead of a Bakelite base. At some point, I watched a Bell System repairman unscrew the varistor cover and screwed in what appeared to be a slide hammer, which he pumped up-and-down for several seconds. To this day, I don't know why he did that.

Older telephone line lightning arrestor

From Wikipedia:

A spark gap is one of the oldest protective electrical technologies still found in telephone circuits, having been developed in the nineteenth century. A carbon rod electrode is held with an insulator at a specific distance from a second electrode. The gap dimension determines the voltage at which a spark will jump between the two parts and short to ground. The typical spacing for telephone applications in North America is 0.076 mm (0.003 inches). Carbon block suppressors are similar to gas arrestors (GDTs) but with the two electrodes exposed to the air, so their behavior is affected by the surrounding atmosphere, especially the humidity. Since their operation produces an open spark, these devices should never be installed where an explosive atmosphere may develop.

There is another photo of a different type on that page.

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    \$\begingroup\$ Do you know what, specifically, causes the carbon blocks (or connections to them) to behave like a diode? I edited my question to include this aspect of deciding upon an answer. I really like your answer so far. \$\endgroup\$ Mar 29, 2020 at 1:41
  • \$\begingroup\$ @MicroservicesOnDDD Thanks, but no I don't. I don't understand enough about them. \$\endgroup\$ Mar 29, 2020 at 1:47
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    \$\begingroup\$ I added the "diodes" tag and "rectification" to the title, as this targets to finding the actual reason or mechanism, which is what I'm after. (I always want to know how something works, and why it happens). Thanks for all your work on my question, and thank you for migrating it. I appreciate all you have done, and your answer especially. Thanks again. \$\endgroup\$ Mar 29, 2020 at 13:41

Telephone line lightning arrestor technology may have progressed from carbon block units to gas discharge tubes and now rest with metal oxide varistors.

The carbon block units, with less than 0.1mm spacing between carbon blocks and a metallic earth electrode, could have caused interference from local AM broadcasts had the gap been bridged by metal deposits caused by arcing over a period of time. In other words, they could have worked as 'razor blade/pencil lead' foxhole radio detectors.

'Improved' gas discharge units may have also contributed to the problem. The 'improved' units had a construction similar to fluorescent tube light starters. Heat from a sustained gas discharge would sufficiently deform a bimetal strip to short circuit the tube electrodes and quench the discharge. The strip would recover its shape on cooling. Failure to recover could have caused improper metallic contact between the electrodes to form a rectifying junction.

Though metal oxide varistors are high voltage semiconductor devices, they exhibit characteristics of back-to-back series-connected Zener diodes and would not cause interference from local AM broadcasts.


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